Light-emitting element

ABSTRACT

In the present invention, a light-emitting element operating at low driving voltage, consuming low power, emitting light with good color purity and manufactured in high yields can be obtained. A light-emitting element is disclosed with a configuration composed of a fist layer containing a light-emitting material, a second layer, a third layer are formed sequentially over an anode to be interposed between the anode and a cathode in such a way that the third layer is formed to be in contact with the cathode. The second layer is made from n-type semiconductor, a mixture including that, or a mixture of an organic compound having a carrier transporting property and a material having a high electron donor property. The third layer is made from p-type semiconductor, a mixture including that, or a mixture of an organic compound having a carrier transporting property and a material having a high electron acceptor property.

TECHNICAL FIELD

The present invention relates to a layered structure of a light-emittingelement that has a layer containing a light-emitting material between ananode and a cathode and that can emit light upon being applied with anelectric field.

BACKGROUND ART

As examples of a photoelectronic device using an organic semiconductormaterial as a functional organic material, a light-emitting element,solar battery, and the like can be nominated. These are devicesutilizing an electrical property (carrier transporting property) or anoptical property (light absorption or light-emitting property) of theorganic semiconductor material. Among others, a light-emitting elementhas achieved remarkable development.

A light-emitting element comprises a pair of electrodes (anode andCathode) and a layer containing a light-emitting material interposedbetween the pair of electrodes. The emission mechanism is as follows.Upon applying voltage through the pair of electrodes, holes injectedfrom the anode and electrons injected from the cathode are recombinedwith each other at an emission center within the layer containing alight-emitting material to lead the formation of molecular excitons, andthe molecular excitons return to the ground state while radiating energyas light. There are two excited states possible from the light-emittingmaterial, a singlet state and a triplet state. It is considered thatlight emission can be obtained through both the singlet state and thetriplet state.

Lately, the reduction of driving voltage is successful (see UnexaminedPatent Publication No. 10-270171) by forming an electron injecting layermade from an organic compound doped with metal having a low workfunction (metal having an electron donor property) such as an alkalimetal, an alkaline earth metal, or a rare earth metal to lower an energybarrier in injecting electrons from a cathode to an organic compound.According to this technique, driving voltage can be reduced despite ofusing stabilized metal such as Al for forming the cathode.

By using the application of the technique, the control of an emissionspectrum of a light-emitting element is successful (see UnexaminedPatent Publication No. 2001-102175). In the Unexamined PatentPublication No. 2001-102175, an electron injecting layer made from anorganic compound doped with metal having electron donor property for theorganic compound is provided. The Unexamined Patent Publication hasdisclosed that the thickness of the electron injecting layer isincreased to vary the optical path between a cathode and alight-emitting layer, so that an emission spectrum emitted to outsidecan be controlled due to the effect of interference of light.

According to the Unexamined Patent Publication No. 2001-102175, theincrease of driving voltage is small by adopting the foregoing electroninjecting layer despite of increasing the thickness of the electroninjecting layer in order to control an emission spectrum. However, infact, unless a peculiar organic compound serving as a ligand such asbathocuproin (BCP) is used, driving voltage is drastically increased.

Therefore, the technique with respect to an electron injecting layerdisclosed in the Unexamined Patent Publications Nos. 10-270171 and2001-102175 has a problem that even if the thickness of the electroninjecting layer is increased to improve a manufacturing yield, or tocontrol an emission spectrum so that color purity is improved, powerconsumption is increased unless an organic compound serving as a ligandis selected to use.

A principle of operation of a light-emitting element disclosed in theUnexamined Patent Publications Nos. 10-270171 and 2001-102175 isexplained hereinafter with reference to FIG. 2.

FIG. 2 illustrates a basic configuration of the conventionallight-emitting element using an electron injecting layer as disclosed inthe Unexamined Patent Publications Nos. 10-270171 and 2001-102175.

In the conventional light-emitting element (FIG. 2), holes injected froman anode 201 and electrons injected from a cathode 204 are recombinedupon being applied with forward bias to emit light within a layercontaining a light-emitting material 202. In this instance, an electroninjecting layer 203 is made from an organic compound doped with metalhaving a high electron donor property for the organic compound (alkalimetal or alkali earth metal).

The electron injecting layer 203 serves for flowing electrons to injectthem to the layer containing a light-emitting material 202. However,since electron mobility of an organic compound is two orders ofmagnitude less than hole mobility of that, driving voltage is increasedif the electron injecting layer is formed to have a thickness comparablein a wavelength of visible light (on the order of submicron) in order,for example, to control an emission spectrum.

DISCLOSURE OF INVENTION

In view of the foregoing, it is an object of the present invention toprovide a light-emitting element capable of being increased itsthickness and operating at low driving voltage by a novel meansdifferent from a light-emitting element using a material serving as aligand in accordance with the prior art. It is more specific object ofthe present invention to provide a light-emitting element that consumeslow power and emits light with good color purity. It is still morespecific object of the present invention to provide a light-emittingelement that consumes low power and is manufactured in high yields.

The inventor found out after their earnest consideration that theforegoing problems can be solved by providing a light-emitting elementhaving the following configuration.

One embodiment of the present invention provides a light-emittingelement comprising an anode and a cathode; and a first layer containinga light-emitting material, a second layer containing n-typesemiconductor, and a third layer containing p-type semiconductor, thelayers each being interposed between the pair of electrodes; wherein thefirst layer, the second layer, and the third layer are sequentiallyformed over the anode to be interposed between the anode and the cathodein such a way that the third layer is formed to be in contact with thecathode.

The n-type semiconductor is preferably metal oxide, specifically, acompound or two or more compounds selected from the group consisting ofzinc oxide, tin oxide, and titanium oxide. The p-type semiconductor ispreferably metal oxide, specifically, a compound or two or morecompounds selected from the group consisting of vanadium oxide, chromiumoxide, molybdenum oxide, cobalt oxide, and nickel oxide.

Another embodiment of the present invention provides a light-emittingelement comprising an anode and a cathode; and a first layer containinga light-emitting material, a second layer containing an organic compoundand a material having an electron donor property, and a third layercontaining p-type semiconductor, the layers each being interposedbetween the pair of electrodes; wherein the first layer, the secondlayer, and the third layer are sequentially formed over the anode to beinterposed between the anode and the cathode in such a way that thethird layer is formed to be in contact with the cathode.

The p-type semiconductor is preferably metal oxide, specifically, acompound or two or more compounds selected from the group consisting ofvanadium oxide, chromium oxide, molybdenum oxide, cobalt oxide, andnickel oxide. In the second layer, the organic compound is preferably anorganic compound having an electron transporting property, specifically,a metal complex having a ligand with a π-conjugated skeleton. Thematerial having an electron donor property is preferably an alkalimetal, an alkali earth metal, or a rare earth metal.

More another embodiment of the present invention provides alight-emitting element comprising an anode and a cathode; and a firstlayer containing a light-emitting material, a second layer containingn-type semiconductor, and a third layer containing an organic compoundand a material having an electron acceptor property, the layers eachbeing interposed between the pair of electrodes; wherein the firstlayer, the second layer, and the third layer are sequentially formedover the anode to be interposed between the anode and the cathode insuch a way that the third layer is formed to be in contact with thecathode.

The n-type semiconductor is preferably metal oxide, specifically, acompound or two or more compounds selected from the group consisting ofzinc oxide, tin oxide, and titanium oxide. In the third layer, theorganic compound is preferably an organic compound having a holetransporting property, specifically, an organic compound having anaromatic amine skeleton. The material having an electron acceptorproperty is preferably metal oxide.

Still more another embodiment of the present invention provides alight-emitting element comprising an anode and a cathode; and a firstlayer containing a light-emitting material, a second layer containing afirst organic compound and a material having an electron donor property,and a third layer containing a second organic compound and a materialhaving an electron acceptor property, the layers each being interposedbetween the pair of electrodes; wherein the first layer, the secondlayer, and the third layer are sequentially formed over the anode to beinterposed between the anode and the cathode in such a way that thethird layer is formed to be in contact with the cathode.

The organic compound is preferably an organic compound having anelectron transporting property, specifically, a metal complex having aligand with a π-conjugated skeleton. The material having an electrondonor property is preferably an alkali metal, an alkali earth metal, ora rare earth metal. Further, the second organic compound is preferablyan organic compound having a hole transporting property, specifically,an organic compound having an aromatic amine skeleton. The materialhaving an electron acceptor property is preferably metal oxide.

Further still more another embodiment of the present invention providesa light-emitting element comprising an anode and a cathode; and a firstlayer containing a light-emitting material, a second layer containing anorganic compound and metal, and a third layer made from metal oxide, thelayers each being interposed between the pair of electrodes; wherein thefirst layer, the second layer, and the third layer are sequentiallyformed over the anode to be interposed between the anode and the cathodein such a way that the third layer is formed to be in contact with thecathode. Alternatively, a light-emitting element is provided thatcomprises an anode and a cathode; and a first layer containing alight-emitting material, a second layer containing an organic compoundand metal, and a third layer containing an organic compound that isdifferent from the foregoing organic compound and metal oxide, thelayers each being interposed between the pair of electrodes; wherein thefirst layer, the second layer, and the third layer are sequentiallyformed over the anode to be interposed between the anode and the cathodein such a way that the third layer is formed to be in contact with thecathode.

The organic compound contained in the second layer is preferably anorganic compound having an electron transporting property, specifically,a metal complex having a ligand with a π-conjugated skeleton. Further,the second organic compound contained in the third layer is preferablyan organic compound having a hole transporting property, specifically,an organic compound having an aromatic amine skeleton. The metal ispreferably an alkali metal, an alkali earth metal, or a rare earthmetal. The metal oxide is preferably a compound or two or more compoundsselected from the group consisting of vanadium oxide, chromium oxide,molybdenum oxide, cobalt oxide, and nickel oxide.

Despite of using sputtering for forming the cathode in thelight-emitting element according to the present invention, alight-emitting element having good characteristics and being sufferedfrom little damage due to the sputtering can be obtained. Therefore, acathode can be formed by using a conductive material that is transparentto visible light such as ITO (Indium Tin Oxide) that is mainly formed bysputtering. In the case of using such the transparent electrode madefrom a conductive material transparent to visible light, alight-emitting element that can emit light from a cathode can beobtained.

By a novel means according to the present invention different from theconventional light-emitting element using a material serving as aligand, a light-emitting element capable of readily being increased itsthickness and operating at low driving voltage can be obtained.Accordingly, a light-emitting element that consumes low power and emitslight with good color purity can be obtained. Simultaneously, alight-emitting element that consumes low power and is manufactured inhigh yields can be obtained.

By using the foregoing light-emitting element for manufacturing alight-emitting device, a light-emitting device that can emit light withgood color purity and consumes low power can be manufactured in highyields.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view of the configuration of a light-emittingelement according to the present invention;

FIG. 2 is an explanatory view of the configuration of a light-emittingelement according to the conventional invention;

FIGS. 3A to 3C are explanatory views of the configuration of alight-emitting element according to the present invention;

FIGS. 4A to 4C are explanatory views of the configuration of alight-emitting element according to the present invention;

FIG. 5 is an explanatory view of a light-emitting device;

FIG. 6 is an explanatory view of the configuration of a light-emittingelement according to the present invention;

FIG. 7 is an explanatory view of the configuration of a light-emittingelement of a comparative example to a light-emitting element accordingto the present invention;

FIG. 8 is an explanatory view of the configuration of a light-emittingelement of a comparative example to a light-emitting element accordingto the present invention;

FIGS. 9A and 9B are explanatory views of a light-emitting device;

FIGS. 10A to 10 C are explanatory views of electric appliances;

FIG. 11 is a view for showing voltage-luminance characteristics of alight-emitting element;

FIG. 12 is a view for showing current-voltage characteristics of alight-emitting element;

FIG. 13 is a view for showing emission spectra of a light-emittingelement;

FIG. 14 is an explanatory view of the configuration of a light-emittingelement according to the present invention;

FIG. 15 is an explanatory view of the configuration of a light-emittingelement of a comparative example to a light-emitting element accordingto the present invention;

FIG. 16 is a view for showing emission spectra of a light-emittingelement; and

FIG. 17 is a view for showing emission spectra of a light-emittingelement.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a principle of operation and a specific configurationexample with respect to embodiments of the present invention areexplained in detail.

A principle of operation of a light-emitting element according to thepresent invention is explained with reference to FIG. 1. FIG. 1illustrates a basic configuration of a light-emitting element accordingto the present invention.

A light-emitting element according to the present invention (FIG. 1) isformed to have a structure in which a first layer 102, a second layer103, and a third layer 104 are formed sequentially over an anode 101 tobe interposed between the anode 101 and a cathode 105. As used herein,the term “anode” refers to an electrode for injecting holes. As usedherein, the term “cathode” refers to an electrode for injectingelectrons or accepting holes.

The second layer 103 is a layer for producing and transportingelectrons. The second layer 103 is made from n-type semiconductor, amixture containing that, or a mixture of an organic compound having acarrier transporting property and a material having high electron donorproperty. Also, the third layer 104 is a layer for producing andtransporting holes. The third layer 104 is made from p-typesemiconductor, a mixture containing that, or a mixture of an organiccompound having a carrier transporting property and a material havinghigh electron acceptor property. Further, the first layer 102 is a layercontaining a light-emitting material and is formed by a single layer ora plurality of layers.

The first layer 102, the second layer 103, and the third layer 104 aremade from selected materials to have selected thicknesses so that anemitting region is formed in the fist layer 102.

Upon applying forward bias to the light-emitting element having theforegoing configuration, electrons and holes are flown out in oppositedirections to each other from the vicinity of an interface between thesecond layer 103 and the third layer 104, respectively as shown inFIG. 1. Among thus produced carriers, electrons are recombined withholes injected from the anode 101 to emit light in the first layer 102.On the other hand, holes are passing through the third layer 104 toreach the cathode 105. In the case of focusing attention on the secondlayer 103 and the third layer 104, reverse bias is applied to a p-njunction. In addition, an amount of generated carriers is notdrastically large but adequate available for operation of thelight-emitting element.

Such the light-emitting element according to the present invention cancontrol an optical path by increasing the thickness of the third layerthat can produce holes and move them. On this point, the light-emittingelement according to the present invention is different from theconventional light-emitting element that controls an optical path byincreasing the thickness of an electron injecting layer 203 containingBCP, that is, a layer that produces electrons and transports them (FIG.2).

Hole mobility of an organic compound generally used as a holetransporting material is higher than electron mobility of an organiccompound generally used as an electron transporting material. Therefore,it is better to increase the thickness of a layer that can move holes(third layer) to control an optical path for preventing driving voltagefrom increasing with the increase of the thickness of the third layer.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading of the followingdetailed description along with the accompanied drawings. The presentinvention is to be understood that various changes and modificationswill be apparent to those skilled in the art. Therefore, unlessotherwise such changes and modifications depart from the scope of thepresent invention hereinafter described, they should be construed asbeing included therein.

Embodiment 1

A light-emitting element according to the present invention is explainedin Embodiment 1 with reference to FIGS. 3A to 3C.

As illustrated in FIGS. 3A to 3C, a light-emitting element has aconfiguration composed of an anode 301 formed over a substrate 300, afirst layer containing a light-emitting material 302 formed over theanode 301, a second layer 303 formed over the first layer 302, a thirdlayer 304 formed over the second layer 303, and a cathode 305 formedover the third layer 304.

As a material for the substrate 300, any substrate as long as it is usedin the conventional light-emitting element can be used. For example, aglass substrate, a quartz substrate, a transparent plastic substrate, ora substrate having flexibility can be used.

As an anode material for forming the anode 301, metal having a largework function (at least 4.0 eV), alloys, compounds having electricalconduction properties, and mixture of these materials are preferablyused. As specific examples of the anode materials, aurum (Au), platinum(Pt), nickel (Ni), tungsten (W), chrome (Cr), molybdenum (Mo), ferrum(Fe), cobalt (Co), copper (Cu), palladium (Pd), nitride of metalmaterial (TiN), or the like can be used besides ITO (indium tin oxide),ITO containing silicon, IZO (indium zinc oxide) composed of indium oxidemixed with zinc oxide (ZnO) of from 2 to 20%.

On the other hand, as a cathode material for forming the cathode 305,metal having a small work function (at most 3.8 eV), alloys, compoundshaving electrical conduction properties, and mixture of these materialscan be preferably used. As specific examples of the cathode materials, atransition metal containing a rare earth metal can be used, besides anelement in the first or second periodic row, that is, an alkaline metalsuch as Li, or Cs, alkaline earth metal such as Mg, Ca, or Sr, alloys ofthese elements (Mg: Ag, Al: Li), or compounds (LiF, CsF, CaF₂).Alternatively, the cathode 305 can be formed by a laminated layerincluding Al, Ag, ITO (including alloys), or the like.

The above anode and cathode materials are respectively deposited byvapor deposition or sputtering to form thin films as the anode 301 andthe cathode 305. These films are preferably formed to have thicknessesof from 10 to 500 nm. A protective layer (barrier layer) may be lastlyformed by an inorganic material such as SiN or an organic material suchas Teflon or styrene polymer. The barrier layer may be eithertransparent or opaque. The barrier layer is formed by the foregoinginorganic material or organic material by vapor deposition, sputtering,or the like.

To prevent the organic layer or the electrode of the light-emittingelement from being oxidized or getting wet, desiccant such as SrOx orSiOx may be formed by electron beam irradiation, vapor deposition,sputtering, sol-gel, or the like.

A light-emitting element according to the invention has the structurethat light generated by recombination of carries within the layercontaining a light-emitting material that serves as the first layer isemitted from either the anode 301 or the cathode 305, or both electrodesto outside as illustrated in FIGS. 3A to 3C (arrows in the drawingindicate emission directions). When light emits from the anode 301 (FIG.3A), the anode 301 is formed by a material having a light transmissionproperty. When light emits from the cathode 305 (FIG. 3B), the cathode305 is formed by a material having a light transmission property. Whenlight emits from both of the anode 301 and cathode 305 (FIG. 3C), theanode 301 and cathode 305 are formed by materials having a lighttransmission property.

The first layer 302 is formed by stacking a plurality of layers. InEmbodiment 1, the first layer 302 is formed by stacking a fourth layer311, a fifth layer 312, and a sixth layer 313. The fourth layer 311 is ahole injecting layer containing a hole injecting material. The fifthlayer 312 is a hole transporting layer containing a hole transportingmaterial. The sixth layer 313 is a light-emitting layer containing alight-emitting material to be provided with an emitting region uponbeing applied with an electric field.

A known material can be used for the layer containing a light-emittingmaterial that serves as the first layer. As the known material, eitherof a low molecular based material or a high molecular based material canbe used.

As a hole injecting material for forming the fourth layer 311, aphthalocyanine compound is useful. For example, phthalocyanine(hereinafter, H₂—Pc), copper phthalocyanine (hereinafter, Cu-Pc), andthe like can be used.

As hole transporting materials for forming the fifth layer 312, anaromatic amine (that is, the one having a benzene ring-nitrogen bond)based compound is preferably used. For example,4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (abbreviated TPD),and derivatives thereof such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated a-NPD) arewidely used. Also used are a star burst aromatic amine compound such as4,4′,4″-tris(N,N-diphenyl-amino)-triphenyl amine (abbreviated TDATA),and 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenyl amine(abbreviated MTDATA). Alternatively, a conductive inorganic compoundsuch as oxide molybdenum, or a composite material of the conductiveinorganic compound and the foregoing organic compound can be used.

As a light-emitting material contained in the sixth layer 313, anorganic compound such as quinacridone, coumarin, rubrene, styryl basedpigments, tetraphenyl-butadiene, anthracene, perylene, coronene,12-phthaloperinone derivative can be used. Further, a metal complex suchas tris(8-quinolinolate) aluminum (hereinafter, Alq₃) can be nominated.

The second layer 303 may be made from n-type semiconductor such as zincoxide, tin oxide, titanium oxide, zinc sulfide, zinc selenide, or zinctelluride. Alternatively, the second layer 303 may include the foregoingn-type semiconductor. Further alternatively, the second layer 303 may beformed by an organic compound doped with a material having an electrondonor property for the organic compound. As the organic compound in thisinstance, an electron transporting material is preferably used, forexample, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviated PBD); the foregoing OXD-7, TAZ, p-EtTAZ, BPhen, BCP can benominated. Besides, a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as Alq₃ that leads to increasing of drivingvoltage conventionally; tris(5-methyl-8-quinolinolate) aluminum(abbreviated Almq₃), or bis(10-hydroxybenzo[h]-quinolinato) beryllium(abbreviated BeBq₂); orbis(2-methyl-8-quinolinolate)-4-phenylphenolato-aluminum (abbreviatedBAlq) can be nominated. On the other hand, as an electron donormaterial, an alkali metal such as Li or Cs; an alkali earth metal suchas Mg, Ca, or Sr; or a rare earth metal such as Er or Yb can benominated. Besides, an organic compound having electron donor property,for example, for Alq₃ such as a tetrathiafulvalene ortetramethylthiafulvalene can be used.

The third layer 304 may be made from p-type semiconductor such asvanadium oxide, chromium oxide, molybdenum oxide, cobalt oxide, ornickel oxide. Alternatively, the third layer 304 may include theforegoing p-type semiconductor. Further alternatively, the third layer304 may be formed by an organic compound doped with a material having anelectron acceptor property for the organic compound. As the organiccompound in this instance, a hole transporting material is preferablyused. Especially, an aromatic amine based compound is preferably used.For example, in addition to TPD, α-NPD that is a derivative of the TPD,or a star burst aromatic amine compound such as TDATA, MTDATA, and thelike can be nominated. On the other hand, as the material having anelectron acceptor property, metal oxide such as molybdenum oxide orvanadium oxide that has an electron acceptor property for α-NPD can benominated. Alternatively, an organic compound having an electronacceptor property for α-NPD such as tetracyanoquinodimethan (abbreviatedTCNQ) or 2,3-dicyanonaphtoquinon (DCNNQ) can be used.

Thus, a light-emitting element according to the present invention can beformed. In this embodiment, a layer made from a material having goodelectron transporting property may be provided to a part of the firstlayer 302 so as to be in contact with the second layer 303. As thematerial having good electron transporting property, a metal complexhaving a quinoline skeleton or a benzoquinoline skeleton such astris(8-quinolinolate) aluminum (abbreviated Alq₃),tris(5-methyl-8-quinolinolate) aluminum (abbreviated Almq₃),bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviated BeBq₂), orbis(2-methyl-8-quinolinolate)-4-phenylphenolato-aluminum (abbreviatedBAlq) is preferably used. Alternatively, a metal complex having anoxazole based or thiazole based ligand such as bis[2-(2-hydroxyphenyl)-benzooxazolate] zinc (abbreviated Zn(BOX)₂), or bis[2-(2-hydroxyphenyl)-benzothiazolate] zinc (abbreviated Zn(BTZ)₂) can beused. In addition to the metal complex,2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviatedPBD); 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl] benzene(abbreviated OXD-7);3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated TAZ);3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated p-EtTAZ); bathophenanthroline (abbreviated BPhen);bathocuproin (abbreviated BCP); or the like can be used.

In the foregoing light-emitting element according to the presentinvention, light can be emitted from the cathode as shown in FIG. 3B inthe case of forming the cathode by a conductive material that istransparent to visible light. Alternatively, light can be emitted fromthe anode as shown in FIG. 3A in the case of forming the anode by aconductive material that is transparent to visible light. Furtheralternatively, light can be emitted from the cathode and anode as shownin FIG. 3C in the case of forming the cathode and anode by a conductivematerial that is transparent to visible light.

As a conductive material that is transparent to visible light and has acomparative high conductive property, the above-mentioned ITO, IZO, andthe like can be nominated. The foregoing materials are generallyunsuitable for forming a cathode.

Since the light-emitting element according to the present invention hasthe structure in which a layer for producing holes and transporting themand a layer for producing electrons and transporting them are provided,driving voltage is not increased even if a material having a high workfunction such as ITO or IZO is used. Therefore, ITO or IZO can be usedas a material for forming the cathode in the light-emitting elementaccording to the present invention.

Despite of using sputtering for forming the cathode in thelight-emitting element according to the present invention, alight-emitting element having good characteristics and being sufferedfrom little damage due to the sputtering can be obtained. Therefore, acathode can be formed by using a conductive material that is transparentto light such as ITO that is mainly formed by sputtering.

Embodiment 2

A configuration of a light-emitting element according to the presentinvention is explained with reference to FIGS. 4A to 4C in Embodiment 2.

A substrate 400, an anode 401, a first layer 402, a second layer 403, athird layer 404, and a cathode 405 can be formed by the same materialsand the same processes explained in Embodiment 1, and are explained inno more details.

As illustrated in FIGS. 4A to 4C, a light-emitting element has aconfiguration composed of a cathode 405 formed over the substrate 400,the third layer 404 formed over the cathode 405, the second layer 403formed over the third layer 404, the first layer 402 containing alight-emitting material formed over the second layer 403, and the anode401 formed over the first layer 402.

A light-emitting element according to the invention has the structurethat light generated by recombination of carries within the first layercontaining the light-emitting material is emitted from either the anode401 or the cathode 405, or both electrodes to outside as illustrated inFIGS. 4A to 4C. When light emits from the anode 401 (FIG. 4A), the anode401 is formed by a material having a light transmission property. Whenlight emits from the cathode 405 (FIG. 4B), the cathode 405 is formed bya material having a light transmission property. When light emits fromboth of the anode 401 and cathode 405 (FIG. 4C), the anode 401 andcathode 405 are formed by materials having a light transmissionproperty.

Thus, the light-emitting element according to the present invention canbe manufactured.

In the foregoing light-emitting element according to the presentinvention, light can be emitted from the anode as shown in FIG. 4A inthe case of forming the anode by a conductive material that istransparent to visible light. Alternatively, light can be emitted fromthe cathode as shown in FIG. 4B in the case of forming the cathode by aconductive material that is transparent to visible light. Furtheralternatively, light can be emitted from the cathode and anode as shownin FIG. 4C in the case of forming the cathode and anode by a conductivematerial that is transparent to visible light.

As described in Embodiment 2, as a conductive material that istransparent to light and has a comparative high conductive property, theabove-mentioned ITO, IZO, and the like can be nominated. The foregoingmaterials are generally unsuitable for forming a cathode since they havea high work function.

Since the light-emitting element according to the present invention hasthe structure in which a layer for producing holes and transporting themand a layer for producing electrons and transporting them are provided,driving voltage is not increased even if a material having a high workfunction such as ITO or IZO is used. Therefore, ITO or IZO can be usedas a material for forming the cathode in the light-emitting elementaccording to the present invention.

Embodiment 3

In this embodiment, a light-emitting element is manufactured over asubstrate 500 such as a glass, quartz, metal, bulk semiconductor,transparent plastic, or flexible substrate. A passive light-emittingdevice can be manufactured by a plurality of such the light-emittingelements over one substrate. A light-emitting element may bemanufactured to be in contact with a thin film transistor (TFT) array asshown in FIG. 5 instead of manufacturing the light-emitting element overthe substrate such as a glass, quartz, transparent plastic, or flexiblesubstrate. Here, reference numeral 511 denotes a TFT; 512, a TFT; and513, a light-emitting element according to the present invention. Thelight-emitting element 513 is composed of an anode 514, first, second,and third layers 515, and a cathode 516; and is connected electricallyto the TFT 511 via a wiring 517. Thus, an active matrix light-emittingdevice that controls driving of a light-emitting element by a TFT can bemanufactured. Further, the structure of the TFT is not especiallylimited. For example, either a staggered TFT or a reverse staggered TFTmay be used. In addition, the crystallinity of a semiconductor layercomposing the is not especially limited. Either a crystalline or anamorphous semiconductor layer may be used.

Example 1

One mode of a light-emitting element according to the present inventionis specifically exemplified in this example. A configuration of thelight-emitting element is explained with reference to FIG. 6.

An anode 601 of the light-emitting element was formed over a substrate600. The anode 601 was made from an ITO that is a transparent conductivefilm by sputtering to have a thickness of 110 nm and a size of 2×2 mm.

Then, a first layer containing a light-emitting material 602 was formedover the anode 601. The first layer containing a light-emitting material602 according to this example was formed to have a layered structurecomposed of three layers, that is, a hole injecting layer 611, a holetransporting layer 612, and a light-emitting layer 613.

A substrate provided with the anode 601 was secured with a substrateholder of a vacuum deposition system in such a way that the surfaceprovided with the anode 601 was down. Then, copper phthalocyanine(hereinafter, Cu-Pc) was put into an evaporation source installed in theinternal of the vacuum deposition system. And then, the hole injectinglayer 611 was formed to have a thickness of 20 nm by vapor depositionwith a resistive heating method. As a material for the hole injectinglayer 611, a known hole injecting material can be used.

The hole transporting layer 612 was made from a material having a goodhole transporting property. As a material for the hole transportinglayer 612, a known hole transporting material can be used. In thisexample, α-NPD was used to have a thickness of 40 nm by the same processas that for forming the hole injecting layer 611.

And then, the light-emitting layer 613 was formed. As a material forforming the light-emitting layer 613, a known light-emitting materialcan be used. In this example, Alq₃ was used to have a thickness of 40 nmby the same process as that for forming the hole transporting layer 612.Here, the Alq₃ serves as a light-emitting material.

A second layer 603 was formed after forming these three layers of thehole injecting layer 611, the hole transporting layer 612, and thelight-emitting layer 613. The second layer 603 was made from Alq₃ as anelectron transporting material and Mg as an electron donor material forthe Alq₃ to have a thickness of 30 nm by co-evaporation in this example.The Mg of 1 wt % was present in the material for the second layer 603.

A third layer 604 was formed. The third layer 604 was made from α-NPD asa hole transporting material and molybdenum oxide as an electronacceptor material for the α-NPD to have a thickness of 150 nm byco-evaporation in this example. The molybdenum oxide of 25 wt % waspresent in the material for the third layer 604. As a raw material forthe molybdenum oxide, molybdenum oxide (VI) was used.

A cathode 605 was formed by sputtering or vapor deposition. The cathode605 was obtained by forming aluminum (150 nm) by vapor deposition overthe third layer 604.

Thus, the light-emitting element according to the present invention wasobtained. FIG. 11 illustrates luminance-voltage characteristics of theobtained light-emitting element. FIG. 12 illustrates current-voltagecharacteristics of the obtained light-emitting element. FIG. 13illustrates an emission spectrum of the obtained light-emitting elementat applied current of 1 mA.

Onset voltage (a voltage for luminance of at least 1 cd/m²) was 6.0 Vupon applying voltage to the obtained light-emitting element. Theluminance of 1130 cd/m² was obtained at applied current of 1 mA. Thelight-emitting element achieved green emission with good color puritywith the CIE chromaticity coordinates x=0.29, y=0.63.

Comparative Example 1

The conventional light-emitting element provided with an electroninjecting layer 703 instead of the foregoing second and third layersaccording to the present invention is specifically explained in thiscomparative example 1. The configuration of the light-emitting elementis explained with reference to FIG. 7. The electron injecting layer 703was, as is the case with the second layer 603 in Example 1, made from anelectron transporting material Alq₃ doped with 1 wt % of Mg that is amaterial having an electron donor property for the Alq₃. The electroninjecting layer 703 was formed to have a thickness of 30 nm as is thecase with the second layer 603 in Example 1. A substrate 700; an anode701; a layer containing a light-emitting material 702 composed of a holeinjecting layer 711, a hole transporting layer 712, and a light-emittinglayer 713; and a cathode 704 were formed to have the same structures asthose in Example 1. Therefore, the light-emitting element according toExample 1 is thicker by the thickness of the third layer 604 (150 nm)than that according to Comparative Example 1.

FIG. 11 illustrates luminance-voltage characteristics of the obtainedlight-emitting element. FIG. 12 illustrates current-voltagecharacteristics of the obtained light-emitting element. FIG. 13illustrates an emission spectrum of the obtained light-emitting elementat applied current of 1 mA. Onset voltage was 5.4 V upon applyingvoltage to the obtained light-emitting element. The luminance of 1360cd/m² was obtained at applied current of 1 mA. The light-emittingelement achieved yellow green emission with not good color purity withthe CIE chromaticity coordinates x=0.34, y=0.58.

The above mentioned results show that the driving voltage (6.0 V) of thelight-emitting element according to Example 1 was almost the same asthat (5.4 V) of the light-emitting element according to ComparativeExample 1, although a total thickness of the light-emitting elementaccording to Example 1 is thicker by 150 nm than that according toComparative Example 1. Further, compared with the emission spectra witheach other in FIG. 13, an emission spectrum according to Example 1 has anarrower spectrum than that of the emission spectrum according toComparative Example 1. Accordingly, it can be considered that the narrowemission spectrum leads to the improvement of color purity of thelight-emitting element according to Example 1.

Comparative Example 2

The conventional light-emitting element provided with a substrate 800;an anode 801; a layer containing a light-emitting material 802 composedof a hole injecting layer 811, a hole transporting layer 812, and alight-emitting layer 813; a cathode 804; each of which has the samestructure as that according to Example 1; and an electron injectinglayer 803 is specifically explained in this comparative example 2. Theconfiguration of the light-emitting element is explained with referenceto FIG. 8. The electron injecting layer 803 has the same structure asthat according to Comparative Example 1. The electron injecting layer803 was formed to have a thickness of 180 nm so that the light-emittingelement has the same total thickness of as that according to Example 1.

FIG. 11 illustrates luminance-voltage characteristics of the obtainedlight-emitting element. FIG. 12 illustrates current-voltagecharacteristics of the obtained light-emitting element. FIG. 13illustrates an emission spectrum of the obtained light-emitting elementapplied with current of 1 mA. Onset voltage was 14.0 V upon applyingvoltage to the obtained light-emitting element. The luminance of 1050cd/m² was obtained at applied current of 1 mA. The light-emittingelement achieved green emission with good color purity with the CIEchromaticity coordinates x=0.25, y=0.63.

Above mentioned results show that the driving voltage of thelight-emitting element according to Comparative Example 2 wasdrastically increased than that of the light-emitting element having thesame thickness according to Example 1 of the present invention althoughthe light-emitting element according to Comparative Example 2 has anarrow emission spectrum as shown in FIG. 13 with good color purity.

Therefore, the practice of one embodiment of the present invention inwhich a first layer 602, a second layer 603, and a third layer 604 aresequentially provided between a pair of electrodes (anode 601 andcathode 605) is enable a light-emitting element to improve color purityby increasing the total thickness of the layers, simultaneously, drivingvoltage to be prevented from being increased despite of increasing thetotal thickness of the layers.

Example 2

One embodiment of a light-emitting element according to the presentinvention is specifically exemplified in this example 2. Theconfiguration of the light-emitting element is explained with referenceto FIG. 14.

An anode 2401 of the light-emitting element is formed over a substrate2400. The anode 2401 was made from an ITO that is a transparentconductive film by sputtering to have a thickness of 110 nm and a sizeof 2×2 mm.

Then, a first layer containing a light-emitting material 2402 was formedover the anode 2401. The first layer containing a light-emittingmaterial 2402 according to this example is formed to have a layeredstructure composed of three layers, that is, a hole injecting layer2411, a hole transporting layer 2412, and a light-emitting layer 2413.

A substrate provided with the anode 2401 was secured with a substrateholder of a vacuum deposition system in such a way that the surfaceprovided with the anode 2401 was down. Then, Cu-Pc was put into anevaporation source installed in the internal of the vacuum depositionsystem. And then, the hole injecting layer 2411 was formed to have athickness of 20 nm by vapor deposition with a resistive heating method.As a material for the hole injecting layer 2411, a known hole injectingmaterial can be used.

The hole transporting layer 2412 was made from a material having a goodhole transporting property. As a material for the hole transportinglayer 2412, a known hole transporting material can be used. In thisexample, α-NPD was used to have a thickness of 40 nm by the same processas that for forming the hole injecting layer 2411.

A light-emitting layer 2413 was formed. As a material for thelight-emitting layer 2413, a known light-emitting material can be used.The light-emitting layer 2413 was formed by co-evaporation of Alq₃ andcoumarin 6 to have a thickness of 40 nm in this example. Here, thecoumarin 6 serves as a light-emitting material. The Alq₃ and thecoumarin 6 were co-evaporated to have a mass ratio of 1:0.003,respectively.

A second layer 2403 was formed after forming the three layers of thehole injecting layer 2411, the hole transporting layer 2412, and thelight-emitting layer 2413. The second layer 2403 was made from Alq₃ asan electron transporting material and Li as a material having anelectron donor property for the Alq₃ to have a thickness of 30 nm byco-evaporation in this example. The Li of 1 wt % was present in thematerial for the second layer 2403.

A third layer 2404 was formed. The third layer 2404 was made from α-NPDas a hole transporting material and molybdenum oxide as a materialhaving an electron acceptor property for the α-NPD to have a thicknessof 180 nm by co-evaporation in this example. The molybdenum oxide of 25wt % was present in the material for the third layer 2404. As a rawmaterial for the molybdenum oxide, molybdenum oxide (VI) was used.

A cathode 2405 was formed by sputtering or vapor deposition. The cathode2405 was obtained by forming aluminum (200 nm) by vapor deposition overthe third layer 2404.

Onset voltage (a voltage for luminance of at least 1 cd/m²) was 3.4 Vupon applying voltage to the obtained light-emitting element. Theluminance of 2700 cd/m² was obtained at applied current of 1 mA. Asillustrated in FIG. 16, an emission spectrum shows a sharp line. Thelight-emitting element achieved green emission with excellent colorpurity with the CIE chromaticity coordinates x=0.21, y=0.69.

Comparative Example 3

In this comparative example 3, the conventional light-emitting elementprovided with an electron injecting layer 2503 instead of the foregoingsecond and third layers according to the present invention isspecifically explained by using FIG. 15. The electron injecting layer2503 was made from an electron transporting material Alq₃ doped with 1wt % of Li that is a material having an electron donor property for theAlq₃ as is the case with the second layer 2503 in Example 2. Theelectron injecting layer 2503 was formed to have a thickness of 30 nm asis the case with the second layer in Example 2. A substrate 2500; ananode 2501; a layer containing a light-emitting material 2502 composedof a hole injecting layer 2511, a hole transporting layer 2512, and alight-emitting layer 2513; and a cathode 2504 were formed to have thesame structures as those in Example 2. Therefore, the light-emittingelement according to Example 2 is thicker by the thickness of the thirdlayer 2404 (180 nm) than that according to Comparative Example 3.

Onset voltage was 3.2 V upon applying voltage to the obtainedlight-emitting element according to Comparative Example 3. The luminanceof 3300 cd/m² was obtained at applied current of 1 mA. As illustrated inFIG. 16, an emission spectrum shows a broad line. The light-emittingelement achieved green emission with not good color purity with the CIEchromaticity coordinates x=0.30, y=0.64.

The above mentioned results show that the light-emitting elementaccording to Example 2 has almost the same driving voltage (3.4 V) asthat (3.2 V) of the light-emitting element according to ComparativeExample 3, although a total thickness of the light-emitting elementaccording to Example 2 is thicker by 180 nm than that according toComparative Example 3. Further, compared with the emission spectra witheach other in FIG. 16, an emission spectrum according to Example 2 has anarrower spectrum than that of emission spectrum according toComparative Example 3. Accordingly, the narrow spectrum leads to theimprovement of color purity of the light-emitting element according toExample 2.

Example 3

One embodiment of a light-emitting element according to the presentinvention is exemplified in Example 3. The configuration of thelight-emitting element is explained with reference to FIG. 14. InExample 3, layers except a third layer 2404 in the light-emittingelement were formed in accordance with the same method conducted inExample 2. Further, the third layer 2404 was made from molybdenum oxideby vapor deposition to have a thickness of 260 nm. As a raw material forthe molybdenum oxide, molybdenum oxide (VI) was used.

Onset voltage (a voltage for luminance of at least 1 cd/m²) was 4.6 Vupon applying voltage to the obtained light-emitting element. Theluminance of 2800 cd/m² was obtained at applied current of 1 mA. Asillustrated in FIG. 17, an emission spectrum shows a sharp line. Thelight-emitting element achieved green emission with excellent colorpurity with the CIE chromaticity coordinates x=0.23, y=0.71. Forcomparison, the above-mentioned emission spectrum of Comparative Example3 is illustrated in FIG. 17.

The above mentioned results show that the driving voltage (4.6 V) of thelight-emitting element according to Example 3 is not increaseddrastically compared as that (3.2 V) of the light-emitting elementaccording to Comparative Example 3, although a total thickness of thelight-emitting element according to Example 3 is thicker by 260 nm thanthat according to Comparative Example 3. Further, compared with theemission spectra with each other in FIG. 17, an emission spectrumaccording to Example 3 has a narrower spectrum than that of emissionspectrum according to Comparative Example 3. Accordingly, the narrowspectrum leads to the improvement of color purity of the light-emittingelement according to Example 3.

Example 4

In this example, a light-emitting device having a light-emitting elementaccording to the present invention in a pixel portion will be explainedwith reference to FIGS. 9A and 9B. FIG. 9A is a top view of alight-emitting device. FIG. 9B is a cross-sectional view of FIG. 9Ataken along line A-A′. Reference numeral 901 indicated by dotted linedenotes a driver circuit unit (a source side driver circuit); referencenumeral 902 denotes a pixel portion; 903, a driver circuit unit (a gateside driver circuit); 904, a sealing substrate; and 905, sealing agent.The inside surrounded by the sealing agent 905 is space 907.

Reference 908 denotes a wiring for transmitting signals to be inputtedto the source side driver circuit 901 and the gate side driver circuit903. The wiring receives video signals, clock signals, start signals, orreset signals from an FPC (flexible printed circuit) 909 serving as anexternal input terminal. Although only the FPC is illustrated in thedrawing, a PWB (printed wirings board) may be attached to the FPC. Asused in this specification, the term “light-emitting device” refers tonot only a main body of a light-emitting device but also the main bodyprovided with the FPC 909 or PWB.

Then, a cross-sectional structure will be explained with reference toFIG. 9B. A driver circuit and a pixel portion are formed over asubstrate 910. In FIG. 9B, the source side driver circuit 901 and thepixel portion 902 are illustrated as a driver circuit unit.

The source side driver circuit 901 is provided with a CMOS circuitformed by combining an n-channel TFT 923 and a p-channel TFT 924. A TFTfor forming a driver circuit may be formed by a known CMOS, PMOS, orNMOS circuit. In this example, a driver integrated type in which adriver circuit is formed over a substrate is described, but notexclusively, the driver circuit can be formed outside instead of over asubstrate.

The pixel portion 902 is composed of a plurality of pixels including aswitching TFT 911, a current control TFT 912, and an anode 913 connectedelectrically to the drain of the current control TFT 912. Further, aninsulator 914 is formed to cover the edge of the anode 913. Here, theinsulator 914 is formed by a positive type photosensitive acrylic resinfilm.

In order to make favorable coverage, an upper edge portion and a loweredge portion of the insulator 914 are formed to have curved faces havingradius of curvatures. For example, in the case that positive typephotosensitive acrylic is used as a material for the insulator 914, onlyupper edge portion of the insulator 914 is preferably having a radius ofcurvature (from 0.2 to 3 μm). As the insulator 914, either a negativetype photosensitive resin that becomes insoluble to etchant by light ora positive type photosensitive resin that becomes dissoluble to etchantby light can be used. For example, not only organic compounds but alsoinorganic compounds such as silicon oxide, silicon oxynitride, orsiloxane can be used.

First to third layers 916, and a cathode 917 are formed over the anode913, respectively. As a material for the anode 913, a material having alarge work function is preferably used. For instance, the anode 913 canbe formed by a single layer such as an ITO (indium tin oxide) film, ITSO(indium tin silicon oxide), an IZO (indium zinc oxide) film, a titaniumnitride film, a chromic film, a tungsten film, a Zn film, or a Pt film;a laminated layer comprising one of the above single layer and a filmcontaining titanium nitride and aluminum as its main components; a threelaminated layer comprising one of the above single layer, a filmcontaining titanium nitride and aluminum as its main components, and atitanium nitride film; or the like. In the case that the anode 913 isformed to have a layered structure, the anode can be formed to have alow resistance as a wiring, and make good ohmic contact, and serve as ananode.

The first to third layers 916 are formed by vapor deposition using anevaporation mask or ink jetting. The first to third layers 916 comprisea first layer containing a light-emitting material, a second layercontaining n-type semiconductor, and a third layer containing p-typesemiconductor. The first, second, and third layers are formedsequentially over the anode to be interposed between the anode and acathode in such a way that the third layer is in contact with thecathode. In addition, as a material for a layer containing alight-emitting material, an organic compound in a single layer, alaminated layer, or a mixed layer is generally used. However, theinvention comprehends the case that a film made from an organic compoundincluding partly an inorganic compound.

As a material for the cathode 917 formed over the first to third layers916, a material having a small work function (Al, Ag, Li, Ca, or alloysof these elements such as MgAg, MgIn, AlLi, CaF₂, or CaN) can be used.In the case that light generated in the first to third layers 916 passesthrough the cathode 917, the cathode 917 is preferably formed to have alayered structure comprising a thin metal film and a transparentconductive ITO (indium tin oxide alloys, ITSO (indium tin siliconoxide), In₂O₃—ZnO (indium zinc oxide), ZnO (zinc oxide), or the like).

The sealing substrate 904 is pasted onto the substrate 910 with thesealing agent 905 to encapsulate a light-emitting element 918 within thespace 907 surrounded by the substrate 910, the sealing substrate 904,and the sealing agent 905. The invention comprehends not only the casethat the space 907 is filled with inert gases (such as nitrogen orargon) but also the case that the space 907 is filled with the sealingagent 905.

Epoxy-based resin is preferably used as the sealing agent 905. Inaddition, it is desirable that the material for the sealing agentinhibits the penetration of moisture or oxygen. As a material for thesealing substrate 904, a plastic substrate such as FRP(fiberglass-reinforced plastics), PVF (poly(vinyl fluoride), Myler,polyester, or acrylic can be used besides a glass substrate or a quartzsubstrate.

Accordingly, a light-emitting device having a light-emitting elementaccording to the invention can be obtained.

The light-emitting device described in this example can be practiced bycombining freely with the configuration of the light-emitting elementexplained in Examples 1 to 3. The light-emitting device according tothis example may be provided with a chromatic conversion film such as acolor filter as needed.

Example 5

Various electric appliances completed by using a light-emitting devicehaving a light-emitting element according to the present invention willbe explained in this example with reference to FIGS. 10A to 10C.

Given as examples of such electric appliances manufactured by using alight-emitting device according to the invention: a television, a camerasuch as a video camera or a digital camera, a goggles-type display (headmount display), a navigation system, a sound reproduction device (a caraudio equipment, an audio set and the like), a personal computer, a gamemachine, a portable information terminal (a mobile computer, a cellularphone, a portable game machine, an electronic book, or the like), animage reproduction device including a recording medium (morespecifically, a device which can reproduce a recording medium such as adigital versatile disc (DVD) and so forth, and includes a display fordisplaying the reproduced image), or the like. FIGS. 10A to 10C showvarious specific examples of such electric appliances.

FIG. 10A illustrates a display device which includes a frame 1001, asupport table 1002, a display portion 1003, a speaker portion 1004, avideo input terminal 1005, and the like. The display device ismanufactured by using the light-emitting device according to theinvention for the display portion 1003. The display device includes allof the display devices for displaying information, such as a personalcomputer, a receiver of TV broadcasting, and an advertising display.

FIG. 10B illustrates a video camera which includes a main body 1301, adisplay portion 1302, a housing 1303, an external connecting port 1304,a remote control receiving portion 1305, an image receiving portion1306, a battery 1307, a sound input portion 1308, an operation key 1309,an eyepiece potion 1310, and the like. The video camera is manufacturedby using the light-emitting device according to the invention for thedisplay portion 1302.

FIG. 10C illustrates a cellular phone which includes a main body 1501, ahousing 1502, a display portion 1503, a sound input portion 1504, asound output portion 1505, an operation key 1506, an external connectingport 1507, an antenna 1508, and the like. The cellular phone ismanufactured by using the light-emitting device according to theinvention for the display portion 1503.

As set forth above, the application range of the light-emitting devicehaving the light-emitting element according to the invention isextremely large. Since the light-emitting element used for thelight-emitting device is formed by using the light-emitting elementaccording to the present invention, the light-emitting device hascharacteristics of operating at low driving voltage and long lifetime.

What is claimed is:
 1. A light-emitting device comprising: an anode; afirst layer containing a light-emitting material over the anode; asecond layer containing an organic compound and one of an alkali metal,an alkaline earth metal, and a rare earth metal over the first layer; athird layer containing a metal oxide; and a cathode in direct contactwith the third layer.
 2. The light-emitting device according to claim 1,wherein the organic compound comprises an electron transportingmaterial.
 3. The light-emitting device according to claim 2, wherein theelectron transporting material is a metal complex having a ligand with aπ-conjugated skeleton.
 4. The light-emitting device according to claim1, wherein the metal oxide comprises at least one compound selected fromvanadium oxide, chromium oxide, molybdenum oxide, cobalt oxide, andnickel oxide.
 5. The light-emitting device according to claim 1, whereinthe cathode is transparent to visible light.
 6. The light-emittingdevice according to claim 5, wherein the cathode comprises at least oneof indium, gallium, tin, and zinc.
 7. The light-emitting deviceaccording to claim 1, wherein the anode and the cathode are transparentto visible light.
 8. The light-emitting device according to claim 7,wherein each of the anode and the cathode comprises at least one ofindium, gallium, tin, and zinc.
 9. A light-emitting device comprising:an anode; a first layer containing a light-emitting material over theanode; a second layer containing a first metal oxide over the firstlayer; a third layer containing an organic compound and a second metaloxide over the second layer; and a cathode in direct contact with thethird layer.
 10. The light-emitting device according to claim 9, whereinthe first metal oxide is different from the second metal oxide.
 11. Thelight-emitting device according to claim 9, wherein the first metaloxide comprises at least one compound selected from zinc oxide, tinoxide, and titanium oxide.
 12. The light-emitting device according toclaim 9, wherein the second metal oxide comprises at least one compoundselected from vanadium oxide, chromium oxide, molybdenum oxide, cobaltoxide, and nickel oxide.
 13. The light-emitting device according toclaim 9, wherein the organic compound comprises a hole transportingmaterial.
 14. The light-emitting device according to claim 13, whereinthe hole transporting material is an organic compound having an aromaticamine skeleton.
 15. The light-emitting device according to claim 9,wherein the cathode is transparent to visible light.
 16. Thelight-emitting device according to claim 15, wherein the cathodecomprises at least one of indium, gallium, tin, and zinc.
 17. Thelight-emitting device according to claim 9, wherein the anode and thecathode are transparent to visible light.
 18. The light-emitting deviceaccording to claim 17, wherein each of the anode and the cathodecomprises at least one of indium, gallium, tin, and zinc.
 19. Alight-emitting device comprising: an anode; a first layer containing alight-emitting material over the anode; a second layer containing afirst organic compound and one of an alkali metal, an alkaline earthmetal, and a rare earth metal over the first layer; a third layercontaining a second organic compound and a metal oxide; and a cathode indirect contact with the third layer.
 20. The light-emitting deviceaccording to claim 19, wherein the first organic compound comprises anelectron transporting material.
 21. The light-emitting device accordingto claim 20, wherein the electron transporting material is a metalcomplex having a ligand with a π-conjugated skeleton.
 22. Thelight-emitting device according to claim 19, wherein the second organiccompound comprises a hole transporting material.
 23. The light-emittingdevice according to claim 22, wherein the hole transporting material isan organic compound having an aromatic amine skeleton.
 24. Thelight-emitting device according to claim 19, wherein the metal oxidecomprises at least one compound selected from vanadium oxide, chromiumoxide, molybdenum oxide, cobalt oxide, and nickel oxide.
 25. Thelight-emitting device according to claim 19, wherein the cathode istransparent to visible light.
 26. The light-emitting device according toclaim 25, wherein the cathode comprises at least one of indium, gallium,tin, and zinc.
 27. The light-emitting device according to claim 19,wherein the anode and the cathode are transparent to visible light. 28.The light-emitting device according to claim 27, wherein each of theanode and the cathode comprises at least one of indium, gallium, tin,and zinc.